The present invention relates to an infrared ray blocking transparent film, for example, a film adhered to a windowpane of a building, an automobile, or the like, primarily in order to block infrared ray in sunlight.
Transparent films having infrared ray blocking properties, transparency (transmissivity) for visible light, and properties of reflection or absorbing of infrared light, have been used for controlling the thermal effects of solar radiation. For example, an infrared ray blocking film is adhered to a windowpane of a building or an automobile or the like so as to reduce heat caused by direct sunlight being transmitted therethrough. In the summer, air conditioning load is reduced by reducing the elevation of room temperature, and in the winter, the heat insulating efficiency at room temperature is improved. As an additional effect, if the windowpane is broken, the scattering of pieces of glass is prevented. In general, such an infrared ray blocking transparent film is formed in such a way that an infrared ray blocking layer is provided on one surface of a base film composed of a synthetic resin such as PET (polyethylene terephthalate) or the like, a hard coat layer as a surface protective layer is laminated on another surface of the base film, and a separate material composed of paper, film, or the like, is adhered via an adhesive layer on the surface of the infrared ray blocking layer. When the film is affixed, the separate material is taken off, whereby the adhesive layer may be adhered to a glass plate.
The infrared ray blocking layer has been formed on a base film by coating various kinds of infrared ray absorbers (for example, immonium, aluminum, and anthraquinone-type compounds) or an infrared ray reflecting material (for example, ZnO, SnO.sub.2, and phthalocyan-type pigments, or the like). However, when the conventional infrared ray absorber is used, the layer is a dark brown or a dark blue, whereby transparency of the layer is inferior since visible light transmissivity is lowered to 50% or less. Alternatively, the layer can absorb only infrared ray having long wavelengths ranging from 1000 to 1500 nm or more, or the layer can absorb only infrared ray having a very small range of wavelengths. Therefore, in order to overcome the above-mentioned defects of the infrared ray absorber and the infrared ray reflecting material, indium tin oxide powder (hereinafter referred to as ITO) which was developed as a conductive coating material, has come into use. In this case, the ITO powder is coated on a base film by a vacuum deposition method or a sputtering method to form a thin film.
The ITO powder may be formed on a base film by a vacuum deposition method or a sputtering method, thereby reflecting infrared ray having wavelengths ranging from 800 to 2500 nm, so that an infrared ray blocking film having high transparency can be provided. However, these methods require a high-vacuum performance apparatus and a high-precision atmosphere controlling system; the cost of production therefore increases and adversely affects mass-productivity. The properties of the method for forming a thin film results in a metallic luster on the reflection surface for visible ray, which is esthetically undesirable even if the film has high transparency. Furthermore, the film has a comparatively low resistance with respect to electromagnetic waves and therefore tends to absorb electromagnetic waves, so that a mobile telephone or a car navigation system occasionally cannot be used inside a vehicle.
Therefore, it is an object of the present invention to provide an infrared ray blocking transparent film which can be produced at low cost even though it employs an ITO powder, which can inhibit occurrence of metallic luster, and which can exhibit excellent electromagnetic wave transmissivity.